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* [PATCH] KVM: arm/arm64: replacing per-VM's per-CPU variable
  2018-03-13 21:03 [PATCH] KVM: arm/arm64: replacing per-VM's per-CPU variable Peng Hao
@ 2018-03-13 13:01 ` Marc Zyngier
  2018-03-13 19:33   ` Robin Murphy
  0 siblings, 1 reply; 4+ messages in thread
From: Marc Zyngier @ 2018-03-13 13:01 UTC (permalink / raw)
  To: linux-arm-kernel

[You're repeatedly posting to the kvmarm mailing list without being
subscribed to it. I've flushed the queue now, but please consider
subscribing to the list, it will help everyone]

On 13/03/18 21:03, Peng Hao wrote:
> Using a global per-CPU variable instead of per-VM's per-CPU variable.
> 
> Signed-off-by: Peng Hao <peng.hao2@zte.com.cn>
> ---
>  arch/arm/include/asm/kvm_host.h   |  3 ---
>  arch/arm64/include/asm/kvm_host.h |  3 ---
>  virt/kvm/arm/arm.c                | 26 ++++++--------------------
>  3 files changed, 6 insertions(+), 26 deletions(-)
> 
> diff --git a/arch/arm/include/asm/kvm_host.h b/arch/arm/include/asm/kvm_host.h
> index 248b930..4224f3b 100644
> --- a/arch/arm/include/asm/kvm_host.h
> +++ b/arch/arm/include/asm/kvm_host.h
> @@ -59,9 +59,6 @@ struct kvm_arch {
>  	/* VTTBR value associated with below pgd and vmid */
>  	u64    vttbr;
>  
> -	/* The last vcpu id that ran on each physical CPU */
> -	int __percpu *last_vcpu_ran;
> -
>  	/*
>  	 * Anything that is not used directly from assembly code goes
>  	 * here.
> diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
> index 596f8e4..5035a08 100644
> --- a/arch/arm64/include/asm/kvm_host.h
> +++ b/arch/arm64/include/asm/kvm_host.h
> @@ -67,9 +67,6 @@ struct kvm_arch {
>  	/* VTTBR value associated with above pgd and vmid */
>  	u64    vttbr;
>  
> -	/* The last vcpu id that ran on each physical CPU */
> -	int __percpu *last_vcpu_ran;
> -
>  	/* The maximum number of vCPUs depends on the used GIC model */
>  	int max_vcpus;
>  
> diff --git a/virt/kvm/arm/arm.c b/virt/kvm/arm/arm.c
> index 86941f6..a67ffb0 100644
> --- a/virt/kvm/arm/arm.c
> +++ b/virt/kvm/arm/arm.c
> @@ -59,6 +59,8 @@
>  /* Per-CPU variable containing the currently running vcpu. */
>  static DEFINE_PER_CPU(struct kvm_vcpu *, kvm_arm_running_vcpu);
>  
> +static DEFINE_PER_CPU(struct kvm_vcpu *, kvm_last_ran_vcpu);
> +
>  /* The VMID used in the VTTBR */
>  static atomic64_t kvm_vmid_gen = ATOMIC64_INIT(1);
>  static u32 kvm_next_vmid;
> @@ -115,18 +117,11 @@ void kvm_arch_check_processor_compat(void *rtn)
>   */
>  int kvm_arch_init_vm(struct kvm *kvm, unsigned long type)
>  {
> -	int ret, cpu;
> +	int ret;
>  
>  	if (type)
>  		return -EINVAL;
>  
> -	kvm->arch.last_vcpu_ran = alloc_percpu(typeof(*kvm->arch.last_vcpu_ran));
> -	if (!kvm->arch.last_vcpu_ran)
> -		return -ENOMEM;
> -
> -	for_each_possible_cpu(cpu)
> -		*per_cpu_ptr(kvm->arch.last_vcpu_ran, cpu) = -1;
> -
>  	ret = kvm_alloc_stage2_pgd(kvm);
>  	if (ret)
>  		goto out_fail_alloc;
> @@ -147,9 +142,7 @@ int kvm_arch_init_vm(struct kvm *kvm, unsigned long type)
>  	return ret;
>  out_free_stage2_pgd:
>  	kvm_free_stage2_pgd(kvm);
> -out_fail_alloc:
> -	free_percpu(kvm->arch.last_vcpu_ran);
> -	kvm->arch.last_vcpu_ran = NULL;
> +out_fail_alloc:
>  	return ret;
>  }
>  
> @@ -179,9 +172,6 @@ void kvm_arch_destroy_vm(struct kvm *kvm)
>  
>  	kvm_vgic_destroy(kvm);
>  
> -	free_percpu(kvm->arch.last_vcpu_ran);
> -	kvm->arch.last_vcpu_ran = NULL;
> -
>  	for (i = 0; i < KVM_MAX_VCPUS; ++i) {
>  		if (kvm->vcpus[i]) {
>  			kvm_arch_vcpu_free(kvm->vcpus[i]);
> @@ -343,17 +333,13 @@ int kvm_arch_vcpu_init(struct kvm_vcpu *vcpu)
>  
>  void kvm_arch_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
>  {
> -	int *last_ran;
> -
> -	last_ran = this_cpu_ptr(vcpu->kvm->arch.last_vcpu_ran);
> -
>  	/*
>  	 * We might get preempted before the vCPU actually runs, but
>  	 * over-invalidation doesn't affect correctness.
>  	 */
> -	if (*last_ran != vcpu->vcpu_id) {
> +	if (per_cpu(kvm_last_ran_vcpu, cpu) != vcpu) {
>  		kvm_call_hyp(__kvm_tlb_flush_local_vmid, vcpu);
> -		*last_ran = vcpu->vcpu_id;
> +		per_cpu(kvm_last_ran_vcpu, cpu) = vcpu;
>  	}
>  
>  	vcpu->cpu = cpu;
> 

Have you read and understood what this code is about? The whole point of
this code is to track physical CPUs on a per-VM basis. Making it global
completely defeats the point, and can result in guest memory corruption.
Please see commit 94d0e5980d67.

Thanks,

	M.
-- 
Jazz is not dead. It just smells funny...

^ permalink raw reply	[flat|nested] 4+ messages in thread
* [PATCH] KVM: arm/arm64: replacing per-VM's per-CPU variable
  2018-03-13 13:01 ` Marc Zyngier
@ 2018-03-13 19:33   ` Robin Murphy
  2018-03-13 21:31     ` Marc Zyngier
  0 siblings, 1 reply; 4+ messages in thread
From: Robin Murphy @ 2018-03-13 19:33 UTC (permalink / raw)
  To: linux-arm-kernel

On 13/03/18 13:01, Marc Zyngier wrote:
> [You're repeatedly posting to the kvmarm mailing list without being
> subscribed to it. I've flushed the queue now, but please consider
> subscribing to the list, it will help everyone]
> 
> On 13/03/18 21:03, Peng Hao wrote:
>> Using a global per-CPU variable instead of per-VM's per-CPU variable.
>>
>> Signed-off-by: Peng Hao <peng.hao2@zte.com.cn>
>> ---
>>   arch/arm/include/asm/kvm_host.h   |  3 ---
>>   arch/arm64/include/asm/kvm_host.h |  3 ---
>>   virt/kvm/arm/arm.c                | 26 ++++++--------------------
>>   3 files changed, 6 insertions(+), 26 deletions(-)
>>
>> diff --git a/arch/arm/include/asm/kvm_host.h b/arch/arm/include/asm/kvm_host.h
>> index 248b930..4224f3b 100644
>> --- a/arch/arm/include/asm/kvm_host.h
>> +++ b/arch/arm/include/asm/kvm_host.h
>> @@ -59,9 +59,6 @@ struct kvm_arch {
>>   	/* VTTBR value associated with below pgd and vmid */
>>   	u64    vttbr;
>>   
>> -	/* The last vcpu id that ran on each physical CPU */
>> -	int __percpu *last_vcpu_ran;
>> -
>>   	/*
>>   	 * Anything that is not used directly from assembly code goes
>>   	 * here.
>> diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
>> index 596f8e4..5035a08 100644
>> --- a/arch/arm64/include/asm/kvm_host.h
>> +++ b/arch/arm64/include/asm/kvm_host.h
>> @@ -67,9 +67,6 @@ struct kvm_arch {
>>   	/* VTTBR value associated with above pgd and vmid */
>>   	u64    vttbr;
>>   
>> -	/* The last vcpu id that ran on each physical CPU */
>> -	int __percpu *last_vcpu_ran;
>> -
>>   	/* The maximum number of vCPUs depends on the used GIC model */
>>   	int max_vcpus;
>>   
>> diff --git a/virt/kvm/arm/arm.c b/virt/kvm/arm/arm.c
>> index 86941f6..a67ffb0 100644
>> --- a/virt/kvm/arm/arm.c
>> +++ b/virt/kvm/arm/arm.c
>> @@ -59,6 +59,8 @@
>>   /* Per-CPU variable containing the currently running vcpu. */
>>   static DEFINE_PER_CPU(struct kvm_vcpu *, kvm_arm_running_vcpu);
>>   
>> +static DEFINE_PER_CPU(struct kvm_vcpu *, kvm_last_ran_vcpu);
>> +
>>   /* The VMID used in the VTTBR */
>>   static atomic64_t kvm_vmid_gen = ATOMIC64_INIT(1);
>>   static u32 kvm_next_vmid;
>> @@ -115,18 +117,11 @@ void kvm_arch_check_processor_compat(void *rtn)
>>    */
>>   int kvm_arch_init_vm(struct kvm *kvm, unsigned long type)
>>   {
>> -	int ret, cpu;
>> +	int ret;
>>   
>>   	if (type)
>>   		return -EINVAL;
>>   
>> -	kvm->arch.last_vcpu_ran = alloc_percpu(typeof(*kvm->arch.last_vcpu_ran));
>> -	if (!kvm->arch.last_vcpu_ran)
>> -		return -ENOMEM;
>> -
>> -	for_each_possible_cpu(cpu)
>> -		*per_cpu_ptr(kvm->arch.last_vcpu_ran, cpu) = -1;
>> -
>>   	ret = kvm_alloc_stage2_pgd(kvm);
>>   	if (ret)
>>   		goto out_fail_alloc;
>> @@ -147,9 +142,7 @@ int kvm_arch_init_vm(struct kvm *kvm, unsigned long type)
>>   	return ret;
>>   out_free_stage2_pgd:
>>   	kvm_free_stage2_pgd(kvm);
>> -out_fail_alloc:
>> -	free_percpu(kvm->arch.last_vcpu_ran);
>> -	kvm->arch.last_vcpu_ran = NULL;
>> +out_fail_alloc:
>>   	return ret;
>>   }
>>   
>> @@ -179,9 +172,6 @@ void kvm_arch_destroy_vm(struct kvm *kvm)
>>   
>>   	kvm_vgic_destroy(kvm);
>>   
>> -	free_percpu(kvm->arch.last_vcpu_ran);
>> -	kvm->arch.last_vcpu_ran = NULL;
>> -
>>   	for (i = 0; i < KVM_MAX_VCPUS; ++i) {
>>   		if (kvm->vcpus[i]) {
>>   			kvm_arch_vcpu_free(kvm->vcpus[i]);
>> @@ -343,17 +333,13 @@ int kvm_arch_vcpu_init(struct kvm_vcpu *vcpu)
>>   
>>   void kvm_arch_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
>>   {
>> -	int *last_ran;
>> -
>> -	last_ran = this_cpu_ptr(vcpu->kvm->arch.last_vcpu_ran);
>> -
>>   	/*
>>   	 * We might get preempted before the vCPU actually runs, but
>>   	 * over-invalidation doesn't affect correctness.
>>   	 */
>> -	if (*last_ran != vcpu->vcpu_id) {
>> +	if (per_cpu(kvm_last_ran_vcpu, cpu) != vcpu) {
>>   		kvm_call_hyp(__kvm_tlb_flush_local_vmid, vcpu);
>> -		*last_ran = vcpu->vcpu_id;
>> +		per_cpu(kvm_last_ran_vcpu, cpu) = vcpu;
>>   	}
>>   
>>   	vcpu->cpu = cpu;
>>
> 
> Have you read and understood what this code is about? The whole point of
> this code is to track physical CPUs on a per-VM basis. Making it global
> completely defeats the point, and can result in guest memory corruption.
> Please see commit 94d0e5980d67.

I won't comment on the patch itself (AFAICS it is rather broken), but I 
suppose there is a grain of sense in the general idea, since the set of 
physical CPUs itself is fundamentally a global thing. Given a large 
number of pCPUs and a large number of VMs it could well be more 
space-efficient to keep a single per-pCPU record of a {vmid,vcpu_id} 
tuple or some other *globally-unique* vCPU namespace (I guess just the 
struct kvm_vcpu pointer might work, but then it would be hard to avoid 
unnecessary invalidation when switching VMIDs entirely).

Robin.

^ permalink raw reply	[flat|nested] 4+ messages in thread
* [PATCH] KVM: arm/arm64: replacing per-VM's per-CPU variable
@ 2018-03-13 21:03 Peng Hao
  2018-03-13 13:01 ` Marc Zyngier
  0 siblings, 1 reply; 4+ messages in thread
From: Peng Hao @ 2018-03-13 21:03 UTC (permalink / raw)
  To: linux-arm-kernel

Using a global per-CPU variable instead of per-VM's per-CPU variable.

Signed-off-by: Peng Hao <peng.hao2@zte.com.cn>
---
 arch/arm/include/asm/kvm_host.h   |  3 ---
 arch/arm64/include/asm/kvm_host.h |  3 ---
 virt/kvm/arm/arm.c                | 26 ++++++--------------------
 3 files changed, 6 insertions(+), 26 deletions(-)

diff --git a/arch/arm/include/asm/kvm_host.h b/arch/arm/include/asm/kvm_host.h
index 248b930..4224f3b 100644
--- a/arch/arm/include/asm/kvm_host.h
+++ b/arch/arm/include/asm/kvm_host.h
@@ -59,9 +59,6 @@ struct kvm_arch {
 	/* VTTBR value associated with below pgd and vmid */
 	u64    vttbr;
 
-	/* The last vcpu id that ran on each physical CPU */
-	int __percpu *last_vcpu_ran;
-
 	/*
 	 * Anything that is not used directly from assembly code goes
 	 * here.
diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
index 596f8e4..5035a08 100644
--- a/arch/arm64/include/asm/kvm_host.h
+++ b/arch/arm64/include/asm/kvm_host.h
@@ -67,9 +67,6 @@ struct kvm_arch {
 	/* VTTBR value associated with above pgd and vmid */
 	u64    vttbr;
 
-	/* The last vcpu id that ran on each physical CPU */
-	int __percpu *last_vcpu_ran;
-
 	/* The maximum number of vCPUs depends on the used GIC model */
 	int max_vcpus;
 
diff --git a/virt/kvm/arm/arm.c b/virt/kvm/arm/arm.c
index 86941f6..a67ffb0 100644
--- a/virt/kvm/arm/arm.c
+++ b/virt/kvm/arm/arm.c
@@ -59,6 +59,8 @@
 /* Per-CPU variable containing the currently running vcpu. */
 static DEFINE_PER_CPU(struct kvm_vcpu *, kvm_arm_running_vcpu);
 
+static DEFINE_PER_CPU(struct kvm_vcpu *, kvm_last_ran_vcpu);
+
 /* The VMID used in the VTTBR */
 static atomic64_t kvm_vmid_gen = ATOMIC64_INIT(1);
 static u32 kvm_next_vmid;
@@ -115,18 +117,11 @@ void kvm_arch_check_processor_compat(void *rtn)
  */
 int kvm_arch_init_vm(struct kvm *kvm, unsigned long type)
 {
-	int ret, cpu;
+	int ret;
 
 	if (type)
 		return -EINVAL;
 
-	kvm->arch.last_vcpu_ran = alloc_percpu(typeof(*kvm->arch.last_vcpu_ran));
-	if (!kvm->arch.last_vcpu_ran)
-		return -ENOMEM;
-
-	for_each_possible_cpu(cpu)
-		*per_cpu_ptr(kvm->arch.last_vcpu_ran, cpu) = -1;
-
 	ret = kvm_alloc_stage2_pgd(kvm);
 	if (ret)
 		goto out_fail_alloc;
@@ -147,9 +142,7 @@ int kvm_arch_init_vm(struct kvm *kvm, unsigned long type)
 	return ret;
 out_free_stage2_pgd:
 	kvm_free_stage2_pgd(kvm);
-out_fail_alloc:
-	free_percpu(kvm->arch.last_vcpu_ran);
-	kvm->arch.last_vcpu_ran = NULL;
+out_fail_alloc:
 	return ret;
 }
 
@@ -179,9 +172,6 @@ void kvm_arch_destroy_vm(struct kvm *kvm)
 
 	kvm_vgic_destroy(kvm);
 
-	free_percpu(kvm->arch.last_vcpu_ran);
-	kvm->arch.last_vcpu_ran = NULL;
-
 	for (i = 0; i < KVM_MAX_VCPUS; ++i) {
 		if (kvm->vcpus[i]) {
 			kvm_arch_vcpu_free(kvm->vcpus[i]);
@@ -343,17 +333,13 @@ int kvm_arch_vcpu_init(struct kvm_vcpu *vcpu)
 
 void kvm_arch_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
 {
-	int *last_ran;
-
-	last_ran = this_cpu_ptr(vcpu->kvm->arch.last_vcpu_ran);
-
 	/*
 	 * We might get preempted before the vCPU actually runs, but
 	 * over-invalidation doesn't affect correctness.
 	 */
-	if (*last_ran != vcpu->vcpu_id) {
+	if (per_cpu(kvm_last_ran_vcpu, cpu) != vcpu) {
 		kvm_call_hyp(__kvm_tlb_flush_local_vmid, vcpu);
-		*last_ran = vcpu->vcpu_id;
+		per_cpu(kvm_last_ran_vcpu, cpu) = vcpu;
 	}
 
 	vcpu->cpu = cpu;
-- 
1.8.3.1

^ permalink raw reply related	[flat|nested] 4+ messages in thread
* [PATCH] KVM: arm/arm64: replacing per-VM's per-CPU variable
  2018-03-13 19:33   ` Robin Murphy
@ 2018-03-13 21:31     ` Marc Zyngier
  0 siblings, 0 replies; 4+ messages in thread
From: Marc Zyngier @ 2018-03-13 21:31 UTC (permalink / raw)
  To: linux-arm-kernel

On Tue, 13 Mar 2018 19:33:55 +0000,
Robin Murphy wrote:
> 
> On 13/03/18 13:01, Marc Zyngier wrote:
> > [You're repeatedly posting to the kvmarm mailing list without being
> > subscribed to it. I've flushed the queue now, but please consider
> > subscribing to the list, it will help everyone]
> > 
> > On 13/03/18 21:03, Peng Hao wrote:
> >> Using a global per-CPU variable instead of per-VM's per-CPU variable.
> >> 
> >> Signed-off-by: Peng Hao <peng.hao2@zte.com.cn>
> >> ---
> >>   arch/arm/include/asm/kvm_host.h   |  3 ---
> >>   arch/arm64/include/asm/kvm_host.h |  3 ---
> >>   virt/kvm/arm/arm.c                | 26 ++++++--------------------
> >>   3 files changed, 6 insertions(+), 26 deletions(-)
> >> 
> >> diff --git a/arch/arm/include/asm/kvm_host.h b/arch/arm/include/asm/kvm_host.h
> >> index 248b930..4224f3b 100644
> >> --- a/arch/arm/include/asm/kvm_host.h
> >> +++ b/arch/arm/include/asm/kvm_host.h
> >> @@ -59,9 +59,6 @@ struct kvm_arch {
> >>   	/* VTTBR value associated with below pgd and vmid */
> >>   	u64    vttbr;
> >>   -	/* The last vcpu id that ran on each physical CPU */
> >> -	int __percpu *last_vcpu_ran;
> >> -
> >>   	/*
> >>   	 * Anything that is not used directly from assembly code goes
> >>   	 * here.
> >> diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
> >> index 596f8e4..5035a08 100644
> >> --- a/arch/arm64/include/asm/kvm_host.h
> >> +++ b/arch/arm64/include/asm/kvm_host.h
> >> @@ -67,9 +67,6 @@ struct kvm_arch {
> >>   	/* VTTBR value associated with above pgd and vmid */
> >>   	u64    vttbr;
> >>   -	/* The last vcpu id that ran on each physical CPU */
> >> -	int __percpu *last_vcpu_ran;
> >> -
> >>   	/* The maximum number of vCPUs depends on the used GIC model */
> >>   	int max_vcpus;
> >>   diff --git a/virt/kvm/arm/arm.c b/virt/kvm/arm/arm.c
> >> index 86941f6..a67ffb0 100644
> >> --- a/virt/kvm/arm/arm.c
> >> +++ b/virt/kvm/arm/arm.c
> >> @@ -59,6 +59,8 @@
> >>   /* Per-CPU variable containing the currently running vcpu. */
> >>   static DEFINE_PER_CPU(struct kvm_vcpu *, kvm_arm_running_vcpu);
> >>   +static DEFINE_PER_CPU(struct kvm_vcpu *, kvm_last_ran_vcpu);
> >> +
> >>   /* The VMID used in the VTTBR */
> >>   static atomic64_t kvm_vmid_gen = ATOMIC64_INIT(1);
> >>   static u32 kvm_next_vmid;
> >> @@ -115,18 +117,11 @@ void kvm_arch_check_processor_compat(void *rtn)
> >>    */
> >>   int kvm_arch_init_vm(struct kvm *kvm, unsigned long type)
> >>   {
> >> -	int ret, cpu;
> >> +	int ret;
> >>     	if (type)
> >>   		return -EINVAL;
> >>   -	kvm->arch.last_vcpu_ran =
> >> alloc_percpu(typeof(*kvm->arch.last_vcpu_ran));
> >> -	if (!kvm->arch.last_vcpu_ran)
> >> -		return -ENOMEM;
> >> -
> >> -	for_each_possible_cpu(cpu)
> >> -		*per_cpu_ptr(kvm->arch.last_vcpu_ran, cpu) = -1;
> >> -
> >>   	ret = kvm_alloc_stage2_pgd(kvm);
> >>   	if (ret)
> >>   		goto out_fail_alloc;
> >> @@ -147,9 +142,7 @@ int kvm_arch_init_vm(struct kvm *kvm, unsigned long type)
> >>   	return ret;
> >>   out_free_stage2_pgd:
> >>   	kvm_free_stage2_pgd(kvm);
> >> -out_fail_alloc:
> >> -	free_percpu(kvm->arch.last_vcpu_ran);
> >> -	kvm->arch.last_vcpu_ran = NULL;
> >> +out_fail_alloc:
> >>   	return ret;
> >>   }
> >>   @@ -179,9 +172,6 @@ void kvm_arch_destroy_vm(struct kvm *kvm)
> >>     	kvm_vgic_destroy(kvm);
> >>   -	free_percpu(kvm->arch.last_vcpu_ran);
> >> -	kvm->arch.last_vcpu_ran = NULL;
> >> -
> >>   	for (i = 0; i < KVM_MAX_VCPUS; ++i) {
> >>   		if (kvm->vcpus[i]) {
> >>   			kvm_arch_vcpu_free(kvm->vcpus[i]);
> >> @@ -343,17 +333,13 @@ int kvm_arch_vcpu_init(struct kvm_vcpu *vcpu)
> >>     void kvm_arch_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
> >>   {
> >> -	int *last_ran;
> >> -
> >> -	last_ran = this_cpu_ptr(vcpu->kvm->arch.last_vcpu_ran);
> >> -
> >>   	/*
> >>   	 * We might get preempted before the vCPU actually runs, but
> >>   	 * over-invalidation doesn't affect correctness.
> >>   	 */
> >> -	if (*last_ran != vcpu->vcpu_id) {
> >> +	if (per_cpu(kvm_last_ran_vcpu, cpu) != vcpu) {
> >>   		kvm_call_hyp(__kvm_tlb_flush_local_vmid, vcpu);
> >> -		*last_ran = vcpu->vcpu_id;
> >> +		per_cpu(kvm_last_ran_vcpu, cpu) = vcpu;
> >>   	}
> >>     	vcpu->cpu = cpu;
> >> 
> > 
> > Have you read and understood what this code is about? The whole point of
> > this code is to track physical CPUs on a per-VM basis. Making it global
> > completely defeats the point, and can result in guest memory corruption.
> > Please see commit 94d0e5980d67.
> 
> I won't comment on the patch itself (AFAICS it is rather broken), but
> I suppose there is a grain of sense in the general idea, since the set
> of physical CPUs itself is fundamentally a global thing. Given a large
> number of pCPUs and a large number of VMs it could well be more
> space-efficient to keep a single per-pCPU record of a {vmid,vcpu_id}
> tuple or some other *globally-unique* vCPU namespace (I guess just the
> struct kvm_vcpu pointer might work, but then it would be hard to avoid
> unnecessary invalidation when switching VMIDs entirely).

The main issue with this approach is to come up with a data structure
that allows quick retrieval/replacement, doesn't have weird locking
requirements and doesn't require to allocate memory unexpectedly.

The advantage of the current approach is to sidestep these potential
issues altogether by having a single, up-front allocation and zero
locking requirements. I fully appreciate that in the long run, we may
need to move away from it. I'm not quite sure what to replace it
with. Something like a per-CPU btree seems like a possibility.

	M.

-- 
Jazz is not dead, it just smell funny.

^ permalink raw reply	[flat|nested] 4+ messages in thread
end of thread, other threads:[~2018-03-13 21:31 UTC | newest]

Thread overview: 4+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2018-03-13 21:03 [PATCH] KVM: arm/arm64: replacing per-VM's per-CPU variable Peng Hao
2018-03-13 13:01 ` Marc Zyngier
2018-03-13 19:33   ` Robin Murphy
2018-03-13 21:31     ` Marc Zyngier

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